1. Field of the Invention
The present invention relates to a method of pretreating an activated carbon for use as a polarized electrode in an electrochemical capacitor such as an electric double-layer capacitor, a pseudo-capacitor, or the like.
2. Description of the Related Art
Materials having large specific surface areas are suitable for use as polarized electrodes in electrochemical capacitors such as electric double-layer capacitors, pseudo-capacitors, or the like. It is customary to use various activated carbons as such materials.
Heretofore, activated carbon is pretreated by being dried at a room or higher temperature under a reduced pressure or vacuum. The pretreated activated carbon is processed into a polarized electrode.
Even when activated carbon is heated at a high temperature under vacuum, some of water strongly adsorbed deeply in pores in the activated carbon cannot be removed. The remaining water in the activated carbon tends to lower the withstand voltage of a resultant electrochemical capacitor, and hence the durability thereof.
It is therefore an object of the present invention to provide a method of pretreating an activated carbon for use as a polarized electrode to remove remaining water from the activated carbon.
To achieve the above object, there is provided in accordance with the present invention a method of pretreating an activated carbon for use as a polarized electrode, comprising the steps of bringing an activated carbon for use as a polarized electrode into contact with an organic compound which forms an azeotropic mixture with water, so that the activated carbon is impregnated with the organic compound, and drying the activated carbon impregnated with the organic compound with heat to remove the azeotropic mixture of the organic compound and the water.
Specifically, before the activated carbon is processed into the polarized electrode, the activated carbon is brought into an organic compound which forms an azeotropic mixture with water. When the activated carbon contacts such an organic compound, the activated carbon is impregnated with the organic compound, which enters deeply into pores in the activated carbon.
Then, when the activated carbon impregnated with the organic compound is heated, the organic compound forms an azeotropic mixture with water adsorbed in the pores in the activated carbon, and the azeotropic mixture is boiled. Therefore, the organic compound and the water can be removed from the activated carbon when the activated carbon is dried with heat.
The activated carbon may be immersed in the organic compound to impregnate the activated carbon with the organic compound. At this time, the activated carbon should preferably be shaped as an electrode for easy handling. For easily treatment, the activated carbon should preferably be impregnated with the organic compound before it is placed in a cell of a capacitor or a battery. However, after the activated carbon is placed in a cell, the cell may be filled with an electrolytic solution that contains the above organic compound. The activated carbon should preferably be preheated in order to remove at least a portion of the water adsorbed by the activated carbon.
In order to remove the water, the organic compound should preferably be easily adsorbed by the activated carbon, and when the organic compound forms an azeotropic mixture with water, the azeotropic mixture should preferably contains a large amount of water, preferably, 28 weight % of water or more. The organic compound may comprise at least one compound selected from the group consisting of benzene, chlorobenzene, ethyl acetate, methyl ethyl ketone, 1,4-dioxane, and methyl acetate. Preferably, the organic compound should comprise benzene because it is selectively adsorbed to sites where water in the activated carbon is adsorbed and can easily form an azeotropic mixture with the water adsorbed to the sites.